The production of liquid crystal displays such as, for example, active matrix liquid crystal display devices (AMLCDs) is very complex, and the properties of the substrate glass are extremely important. First and foremost, the glass substrates used in the production of AMLCD devices need to have their physical dimensions tightly controlled. The downdraw sheet drawing processes and, in particular, the fusion process described in U.S. Pat. Nos. 3,338,696 and 3,682,609, both to Dockerty, are capable of producing glass sheets that can be used as substrates without requiring costly post-forming finishing operations such as lapping and polishing. Unfortunately, the fusion process places rather severe restrictions on the glass properties, which require relatively high liquidus viscosities.
In the liquid crystal display field, thin film transistors (TFTs) based on poly-crystalline silicon are preferred because of their ability to transport electrons more effectively. Poly-crystalline based silicon transistors (p-Si) are characterized as having a higher mobility than those based on amorphous-silicon based transistors (a-Si). This allows the manufacture of smaller and faster transistors, which ultimately produces brighter and faster displays.
One problem with p-Si based transistors is that their manufacture requires higher process temperatures than those employed in the manufacture of a-Si transistors. These temperatures range from 450° C. to 600° C. compared to the 350° C. peak temperatures employed in the manufacture of a-Si transistors. At these temperatures, there are several properties of the glass composition that need to be taken into consideration. For example, the coefficient of thermal expansion (CTE) should be in a range such that there is minimal distortion to silicon transistors during cool-down from the high-temperature annealing step. It is desirable to minimize the CTE of the glass composition. Other properties of the glass composition to consider include the density and Young's modulus, which contribute to the propensity of the glass sheet to sag. The sheet geometry is dictated by the particular process employed, which is beyond the control of the glass manufacturer. For fixed density, an increase in Young's modulus is desirable since it reduces the amount of sag exhibited by large glass sheets during shipping, handling and thermal processing. Likewise, any increase in density should be accompanied by a proportionate increase in Young's modulus or it will result in increased sag. Thus, glass compositions with a low CTE and high specific modulus (i.e, low density and high Young's modulus) are desirable.
Other properties of the glass composition if not in the proper range can adversely affect the glass-making process. For example, if the glass has 200 poise temperature that is very high, this creates a problem for premelt refractories and possible erosion of Pt/Rh in the finer. If the glass has a high stir, the formation of Pt inclusions in the glass are possible. Additionally, if the delivery temperature of the glass is high, this can present a problem for isopipe corrosion and sag. Finally, the use of chemical fining agents are needed to control liquidus temperature, liquidus viscosity and liquidus phase (cristobalite) for the fusion draw process. However, chemical fining agents are limited in their ability to control these properties. Moreover, chemical fining agents such as, for example, arsenic, are generally not preferred in the glass-making process due to environmental concerns.
Described herein are alkali-free glasses and methods for making the same that possess a number of desirable properties required for downdraw processing, which is important in the manufacturing of substrates for liquid crystal displays.